1. Field of the Invention
The present invention relates to an active noise cancellation apparatus for actively reducing road noise.
2. Description of the Related Art
It is known that road noise is generated by road surface vibration being transmitted and causing a floor panel or other plate-like interior parts (e.g., a windshield, a rear window, door panels) to vibrate. Some apparatus for actively reducing this road noise are disclosed in Japanese Unexamined Patent Publication Nos. H07-281676 and H07-210179. The apparatus disclosed in Japanese Unexamined Patent Publication No. H07-281676 comprises a vibration generator mounted on a floor panel, uses vibration of a suspension system as a reference signal and vibration of the floor panel as an error signal, and controls the vibration generator so as to reduce the error signal. The patent literature states that road noise can be reduced by suppressing vibration of the floor panel by this vibration generator control.
The apparatus disclosed in Japanese Unexamined Patent Publication No. H07-210179 reduces road noise by detecting acceleration as a reference signal by acceleration sensors mounted on a suspension and a vehicle body, detecting sound pressure in a vehicle interior as an error signal by a microphone, and outputting control sound (secondary sound) by a speaker placed in the vehicle interior.